Now, I'm not an expert, so if this post makes you laugh, you're welcome. Still, as far as I know there are basically two components that determine the potential quality of a camera's photos:

Sensor

Lens(es)

I know that sensor technology is still improving over the years, but what about lenses? Is there any development in that area (e.g. smoother glass or something), or has lens technology already been perfected before the digital age?

5 Answers
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Yes. There is development in four areas: computer design, material science, features, and finally a category I'm going to call "not better just different".

Computer Design

Lens design has always been a mix of art and science. In the first part of the previous century, art was clearly primary (even for scientific lens designers). Now, lens design software shifts the balance towards science. There's certainly still art involved in making a lens with pleasing rendering, but the science sure helps. Every lens is a compromise between different constraints: optical (aberrations, sharpness, telecentricity, zoom, parafocal vs.varifocal), physical (number of elements, size and weight), and cost (type of elements used, build quality, complication). Software helps designers create a lens within predetermined acceptable criteria, and it lets them test that lens using simulation before spending a lot of money to determine if the concepts are sound.

This software follows both general improvements in the design software (as one might see improvements in Photoshop or in any CAD program), and developments in the fields of optics and photonics. The same advances in computational photography that enable the Lytro light-field camera help out here. And these advances in software in turn are reflected in the modern lens designs created this way.

I'm going to lump improvements in manufacturing in with this category; maybe it deserves its own. Modern manufacturing techniques use computerized machinery to reliably produce complicated individual lens elements, making their use less expensive where they might have been prohibitively costly before.

Material Science

There are three big areas where this is important.

First, the glass. Different compositions of glass have different optical properties, with varying desirability for photographic lenses — for example, low refractive index, low dispersion, and high light transmittance are all good. Many of the old ways of making glass with desirable properties were quite expensive or have other serious drawbacks. Advances in material science have produced glass with similar properties without those downsides. It's likely this will continue to be the case.

Second, the coatings on the lenses have improved. These are used on all good lenses to reduce glare, which is very important because stray light bouncing around reduces image quality. Newer coatings do this better, more cheaply, and have other desirable properties like repelling fingerprints and dust.

And third: plastics! We're not at the point where plastic elements can replace glass in anything but toy lenses, but plastic is used increasingly in lens construction where metal would be before. In some cases, this is just to make them cheaper with no concern for quality, but when good plastics are used well, they can make a lens lighter and smaller with no compromise to build quality.

Features

I'm going to highlight image stabilization, since that's the obvious one. Modern lenses can offer up to five stops of benefit from stabilization — that is, shutter speeds of up 32× longer with the same sharpness. And newer advances in IS correct for more complicated and different types of movement. Since competition here is fierce and there are a lot of ideas still untapped, expect this area to continue to improve rapidly.

As bokeh — the visual quality of the out-of-focus areas — has become an increasingly important factor, a higher number of a aperture blades and blades with rounded edges are more common. This feature has been available for a long time on premium portrait-lens designs, but now it seems to be almost a must-have feature even on a lower-end "nifty fifty", like those from Nikon and Pentax.

Another example is better in-lens motors, using ring-type ultrasonic designs. And yet another example is the clutch mechanism in newer Pentax lenses, which allows full-time manual focus even with body-driven autofocus. Or, some Pentax lenses have a clever built-in/pull-out lens hoood. This isn't anything to do with optical design, but is an example of practical innovation which is really beneficial to the photographer.

Weather-sealing is another feature: there's nothing particularly innovative about that (except some of the material science, perhaps), but fitting it into more lens designs is progress.

As there's more convergence between video and still photography, we'll see some more changes related to that: more silent operation, and stepless aperture settings (rather than being limited to the traditional stops or predetermined fractions thereof; this allows smooth changes while filming without causing jumps in exposure). Arguably many of these features fall into the next category when viewed from the non-video perspective, as for example stepless aperture isn't really a feature with a lot of benefit for still photography.

Not Better, Just Different

In this category: changes made to benefit digital, and new designs for smaller sensors.

For digital, designs need to take into account the increased reflectance of sensor material over film. This means that there's more stray light bounced back into the lens than there was before. Additionally, most sensors are less forgiving of light that isn't coming from straight-on, making telecentric design more important.

And, smaller sensors simply means that lenses can be designed with a smaller image circle, or at least with those important properties only optimized for the center without worrying so much about what would be the corners on full-frame. This allows smaller, lighter, and cheaper designs which still offer excellent image quality — Pentax's DA Limited series being the poster-child here, with the smc DA 15mm f/4 ED AL Limited being an example of a recent innovative lens design which incorporates many of the things I've listed above.

There's another change which could be put in this category as well. Many cameras now offer automatic software correction of lens defects like chromatic aberration and barrel distortion. In fact, in some point & shoot and compact interchangeable lens cameras, this isn't even optional — it's just on. The camera communicates electronically with the lens and "knows" how to adjust the image in RAW processing in order to compensate for that lens model's particular quirks. This allows the compromise parameters for the lens design to be different: those factors which can easily be corrected for in software can be left to go wild, and other desired characteristics taken beyond what they could be otherwise. Right now, the focus is mainly on size, weight, and cost, but as image processing gets faster and better, it won't be too surprising to see this thinking come into high-end designs as well.

In regard to the "Not Better, Just Different" category, I really hope the major brands come out with some nice APS-sized primes soon. I could really go for a 24mm f/2.
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Evan KrallDec 20 '11 at 7:29

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There's also more to bokeh than iris shape; a lot of work in modern lens design goes into making sure that the out-of-focus circle of confusion is a fuzzy dot rather than a sort of ring shape (nisen-bokeh). The Undisputed King of Cream, the Sony (Minolta) 135mm f/2.8 [T/4.5] STF gives up a stop-and-a-third of light in order to insert an apodization element, a sort of radially-graduated ND filter on steroids, to ensure the bokeh has the right characteristics (along with the 10-bladed iris). Newer lenses try to accomplish the same thing without the exotic element and light loss.
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user2719Aug 13 '12 at 17:35

@StanRogers — oh, absolutely. I didn't mean to give the impression that blades were all there is to bokeh, just that that's the driver for that trend.
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mattdmAug 13 '12 at 19:14

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@mattdm -- that was meant more as an annotation than a criticism (as are most of my comments; I hope people understand that). I just hate editing things into good answers -- I don't need to see my smilin' face on the cover of every issue of the Rolling Stone (there's something about the co-credit for the last editor that just plain bugs me).
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user2719Aug 13 '12 at 20:15

I'd consider the last paragraph to be properly "better": using software to gain more freedom in lens design, or even cut away one of several competing dimensions seems to be incredible progress.
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RaphaelJul 9 '14 at 16:00

Speaking from the world of amateur astronomy, there's quite a bit of development happening with lenses. Eyepieces and objectives are all using new, exotic glass and computing resources to design well-corrected refractive devices. New glass mixes don't come along very often and the proper effort to better mate the shapes and characteristics still requires technical skill.

A recent newsletter from Stellarvue (a US telescope manufacturer) discusses the way they work with the different types of glass available to give new twists to older designs. Stellarvue Newsletter

Oh, and if you know an amateur astronomer, mention "Ethos" eyepieces to them. They're a recent design that's become all the rage.

From the world of photography, there's been recent offerings to have newer lens designs that give more light at a straight-on angle at the CCD/CMOS vs the older film designs that could accommodate the different angles. These lenses tend to be called "digital" or digital ready as they're built to give better illumination for the CCD wells.

Additionally, recent offerings have included lenses that take advantage of only having to illuminate a smaller image circle to cover smaller sensors. This allows making the overall lenses smaller and less expensive.

For a survey of how things have changed within a specific brand, you can browse Bojidar Dimitrov's Pentax K-Mount Page. I suggest looking at how the design for the primes and zooms have changed over the years. Still, it would take an optical engineer to describe the details of how and why certain changes were made.

Aside from optical design, anti-reflective coatings have also improved over the years. They've become more durable and more efficient.

EDIT: As to the organic sensor I only know that Fuji will make it, has the name suggests it uses organic components and Fuji claims that this sensor can surpass the quality of any other FF sensor, more information here.

Finally the global shutter, quoting:

a Global Shutter captures all of the information from every photosite on a chip all at once

this means that it can remove the typical effects from current sensors using the rolling shutter like exhibit smearing, skew, wobble, and partial exposure. More info here.

There are certainly still improvements being made to lenses. The quality control of the glass being produced has improved over time, and also there have been a lot of improvements made to coatings to reduce reflections/glare.

Besides the actual lens elements, there are also a lot of other components of a lens that are still being developed, on great example is IS/VR systems, which have become far more intelligent in recent years.

For a good overview of some of the high-end technology that goes into lenses check out the Canon L Series page. (Yes is is more or less just a big ad for canon, but the page does contain a lot of interesting information.)

Nikon has a Optics Technology section that has a lot of good info as well, but not as much history (and not quite as flashy).